A method and apparatus for non-invasively determining a concentration of glucose in a subject using optical excitation and detection is provided. The method includes emitting an exciter beam (B1) to irradiate a portion (130) of tissue of the subject, causing physical and chemical changes in the surface, and causing an initial back scattering (D1) of light. The method further includes periodically emitting a probe beam (B2) which irradiates the portion of tissue and causes periodic back scatterings (D2) of light. The initial and periodic back scatterings are detected and converted into electrical signals of at least the amplitude, frequency or decay time of the physical and chemical changes, the back scatterings being modulated by the physical and chemical changes. By differentiating over time at least one of the amplitude, frequency or decay time of the physical and chemical changes, the concentration of glucose may be determined.